Honey bee flying with pollen – Photo by Alex Wild, used with permission

Honeybee colonies are famous for their orderly divisions of labour. As worker bees grow up, they transition from housekeepers (cleaning the colony) to nurse bees (feeding young bees), to finally switching to foragers who go out and collect nectar and pollen for the rest of the colony. To maintain a healthy colony, bees need to decide how many foragers and how many nurse bees are needed, and control of these numbers is accomplished by pheromone levels within the colony.

In honeybee colonies, there are pheromones like the alarm pheromone that cause immediate behavioural responses (called releaser pheromones) and others that trigger physiological changes like hormones do (called primer pheromone). From previous work, it seemed that ethyl oleate functions as a primer pheromone, produced by foragers, that delays the maturation of nurse bees into foragers.

“Ethyl oleate does not elicit any noticeable behavourial responses in recipient workers,” says Dr. Erika Plettner, who supervised a recent study on the synthesis of ethyl oleate at Simon Fraser University in British Columbia. “Yet it has a profound physiological effect”.

To understand how this chemical is produced in the individual bee and then distributed in the colony, Carlos Castillo and colleagues from Simon Fraser University in British Columbia and the Laboratoire Biologie et protection de L’Abeillie in France looked at several ways to identify the source and synthesis of ethyl oleate. This chemical can be produced by a reaction between oleic acid (a common fatty acid in insects) and ethanol. While you might not think of honeybees as heavy drinkers, it turns out that yeasts in flower nectar ferment the sugars present into ethanol, and so the forager bees have much higher exposure to ethanol than nurse bees.

To figure out if ethanol and oleic acid can be made into ethyl oleate by honeybees, the researchers incubated different honeybee body parts from forager and nurse bees with these precursors. They found highest production of ethyl oleate in the head tissues, and that both nurses and foragers could produce ethyl oleate when given ethanol. In addition, in whole bees, they found that the ethyl oleate migrated from the gut to the exoskeleton of the bees where it would exude into the colony.

Taken together, these results suggest that making ethyl oleate, while it is useful for colony control, might also be a way to deal with the occupational hazard of consuming toxic ethanol. “Foragers have much higher occupational exposure to ethanol than nurses do,” says Dr. Plettner. “This is why they make ethyl oleate in nature”.

Ethyl oleate

To track down where exactly the ethyl oleate was synthesized, they coupled oleic acid to a chemical that would produce fluorescence when the oleic acid was combined with ethanol to produce ethyl oleate. Under the microscope, areas that fluoresced showed where ethyl oleate was being made. They found that ethyl oleate was made in the esophagus, honey crop and stomach.

The authors were also able to identify the genes responsible for the synthesis of ethyl oleate in the honeybee and the resulting enzymes that catalyze the reaction between oleic acid and ethanol. These enzymes are then secreted into the gut fluid, where they produce ethyl oleate, which is then transported to the cuticle.

The biosynthesis of ethyl oleate then can be thought of a way of providing updates to the colony about the availability of flower nectar in nature. “EO might be some kind of ‘resource meter’ that tells the nurses in the colony how many nectar and pollen resources are coming in,” says Dr. Plettner. “If lots of food is coming in, then it makes sense to inhibit nurse to forager transition, as the nurses would be more needed in the brood chamber than as foragers. Conversely, if few resources and/or foragers are coming in, then it makes sense to speed up development of nurses so that they can forage and fill the need.”